Motors, particularly electrical motors, play a key role in industry. Such motors are used to drive fans, pumps, compressors, valves, and many other machines. It is potentially very costly to allow a significant problem to go on unnoticed in either the motor or the motor driven machine. It is also costly and very time consuming to attempt to repair a nonexistent problem. With present less definitive methods, these costly situations often occur. Thus there is a need for an improved diagnostic method and apparatus as described in the invention for use with motors and motor driven machines.
Present conventional techniques have been developed to monitor such motor systems. One such commonly used conventional technique involves analyzing the signature developed from a single motor current probe in order to ascertain motor fault conditions. However, there are shortcomings associated with this and other standard motor monitoring techniques which prevent optimal motor analysis.
A first problem with conventional current probe technology for motor system analysis is that current and voltage envelopes are generally estimated using inexact demodulation techniques such as rms to D.C. conversion or rectification, followed by low-pass filtering. These signal processing methods exhibit a slow response to rapidly changing current or voltage signals since they rely heavily on low-pass filtering. This reliance results in an inaccurate response to highly dynamic current features such as inrush current.
A second problem with such conventional technology for motor analysis is non-linearity in the current probes, especially at the lower part of their detection range. This problem is often dealt with by specifying a lesser accuracy at low currents, changing the specification from percent reading to percent full scale, or simply not specifying the accuracy over a portion of the detection range.
A third problem with such conventional technology is the distorting influence of filters, non-simultaneous analog-to-digital converter sampling, and other hardware-induced signal processing errors in which the true phase relation between measurement channels is impaired.
A fourth shortcoming of some conventional methods is the need to take the motor off-line in order to ascertain resistive and inductive imbalance. This is a great inconvenience when dealing with continuous-duty motors. In addition, the off-line measurements may not be truly representative of the on-line resistive or inductive balance, since operational stresses may significantly change the state of balance.
A fifth shortcoming with present conventional technology is the exclusive reliance on electrical measurements alone to monitor and diagnose all problems in such motor systems. A motor is not just electrical; it is an electromechanical device, encompassing as well the mechanical aspects of the driven machine, and analyzing either the electrical or the mechanical aspects in isolation ignores important monitoring and diagnostic information.
The present invention overcomes many of the problems of present conventional monitoring techniques by providing a method and apparatus which improves the quality of voltage/current envelope estimation by utilizing an analytic signal approach in which the signal is uniquely defined in terms of quadrature components, and to do so for polyphase motors so that the estimation of polyphase variables such as motor power is improved as well as that of parameters primarily associated with individual phases, such as voltage and current. The present method and apparatus overcomes the non-linear effects of a current probe by modeling the true probe response in software, estimating the model parameters using actual calibration data, and subsequently correcting all measured current values utilizing the model. The present method and apparatus also overcomes the effect of sampling skew and other measurement channel time delay effects by accurately estimating channel-to-channel phase delays using an analytic signal approach in which quadrature components are used to calculate the instantaneous phases of the multi-channel signals, and the instantaneous values are statistically combined to yield accurate cross-channel phase corrections. In addition, the present method and apparatus provides a measure of inphase and reactive imbalance during operation by comparing the voltage and current phasors. Finally, the present method and apparatus enhances the monitoring and diagnostic capability of motor monitoring techniques by simultaneously measuring electrical and mechanical variables, using both types of variables together to more definitively diagnose the existence of certain faults, some of which both types of measurements are sensitive to, and some of which only electrical or only mechanical measurements are sensitive to.